(a) Field of the Invention
The present invention relates to a water-in-oil emulsion explosive composition, and more particularly relates to a water-in-oil emulsion explosive composition containing a cushioning medium, which has a high absorbing property of impact energy, and having a remarkably high resistance against dead pressing.
(b) Description of Related Art
Various investigations have been recently made with respect to water-in-oil emulsion explosive (hereinafter, abbreviated as W/O explosive). As disclosed in U.S. Pat. Nos. 3,161,551, 3,447,978, 3,765,964, 3,674,578, 4,218,272, 4,110,134, 315,784, 4,315,787 and the like, the W/O explosive has a water-in-oil emulsified microfine structure consisting of a continuous phase, which consists mainly of a carbonaceous fuel component such as mineral oil, wax or the like, a disperse phase, which consists of an aqueous solution of inorganic oxidizer salt, such as ammonium nitrate or the like, an emulsifier for forming and maintaining an emulsified microfine structure, and a density adjusting agent for maintaining the detonability of the explosive. That is, the W/O explosive has an emulsified structure which is completely reverse to that of hitherto been known oil-in-water slurry explosive (hereinafter, abbreviated as O/W explosive). Due to the difference in the emulsified microfine structure, W/O explosive is different from O/W explosive in the composition and performance, and W/O explosive is superior to O/W explosive in the contact efficiency between the carbonaceous fuel component and the inorganic oxidizer salt, and hence the W/O explosive has various excellent properties. For example, the W/O explosive has a high detonation velocity, has a detonability in itself without containing a sensitizer, is good in after-detonation-fume and excellent in water-resistance, and can be adjusted widely its consistency (Kogyo Kayaku Kyokai-Shi, Vol. 43 (No. 5), pages 285-294 (1982)).
However, in order to maintain the detonability of a W/O explosive, and to ensure the cap-sensitivity and the sympathetic detonability by a booster in the W/O explosive, the use of a density adjusting agent is indispensable in order to contain bubbles in the explosive and adjust its density.
As the density adjusting agent, there have hitherto been commonly used hollow microspheres which contain an independent bubble (the above described U.S. patents, and U.S. Pat. Nos. 4,326,900, 4,398,976 and 4,414,044 and Japanese patent Laid-open application No. 158,194/80).
As the hollow microspheres, there have been used relatively rigid hollow microspheres, such as glass hollow microspheres, silica hollow microspheres, polyvinylidene chloride hollow microspheres and the like, which have a particle size of about 10-175 .mu.m, and form predominantly a single independent bubble and have a theoretical density of not higher than 0.5 g/cm.sup.2.
The hollow microspheres are used in a W/O explosive in order that a bubble contained in the hollow microspheres is acted as a hot spot in the W/O explosive, whereby the density of the W/O explosive is adjusted in order to give the detonability to the explosive.
Accordingly, it is clear from the disclosure of the working example in any of the above described U.S. patent specifications that the hollow microspheres are relatively rigid hollow microspheres, which form a single independent bubble and have a particle size of 10-175 .mu.m.
There are common unsolved problems in water-gel explosives, such as W/O explosive, O/W explosive and the like, containing these conventional hollow miscrospheres, which form a single independent bubble, as a density adjusting agent.
That is, when it is intended to initiate a water-gel explosive, hollow microspheres contained in the explosive are broken due to the shock, gas pressure, rock pressure and the like, which have been generated by the preceding blasting of an explosive charged in an adjacent bore hole, whereby the former explosive loses its detonability, that is, the dead pressing phenomenon occurs in the former explosive.
Further, when a water-gel explosive is used in the form of a small-diameter cartridge or is blasted in a long blasting hole, the water-gel explosive is often encountered with an interruption of detonation. This phenomenon is known as a so-called channel phenomenon, wherein, when a chain of water-gel explosives charged in one bore hole are detonated, compressed gas consisting of the generated high pressure gas goes ahead of the detonation wave, and compresses a water-gel explosive, which has not yet been detonated, and as the result the compressed gas breaks hallow microspheres contained in the water-gel explosive and makes the explosive to lose its detonability (Hanazaki et al, "Kogyo Kayaku Kyokai-Shi", 45(3), 149-155 (1984)).
These two phenomena are common to each other in the point that hollow microspheres contained in a water-gel explosive are broken by the high pressure acted from the exterior, and the density of the explosive is increased and the detonability thereof is lost.
In order to improve the ability for maintaining the detonability, that is, to improve the resistance against dead pressing, hollow microspheres having a high breaking strength are generally used (Japanese Patent Laid-open Application No. 51,686/85).
However, it is necessary to use a material having a higher hardness and to make the shell thickness into a larger thickness in order to improve the strength of hollow microspheres. The use of hard hollow microspheres having a large shell thickness results in hollow microspheres having a high theoretical density. Therefore, a large amount of expensive hollow microspheres must be used in order to adjust the density of a water-gel explosive to a given density (generally 1.20 or less), which density is necessary to maintain the cap-sensitivity of the water-gel explosive. As the result, the use of a large amount of expensive hollow microspheres is not desirable in view of inexpensive production of water-gel explosive, and further causes deteriorations of explosion strength, storage stability and sympathetic detonability in the resulting water-gel explosive. While, when hollow microspheres having a very high hardness are used, although the resistance of the resulting water-gel explosive against dead pressing is somewhat improved, the external pressure, which causes dead pressing phenomenon and channel phenomenon, in the resulting water-gel explosive is higher than the breaking strength of the hollow microspheres, and therefore the resistance of water-gel explosive against dead pressing can not be satisfactorily improved by a conventional method, wherein a density adjusting agent having a higher strength is merely used.
Further, there have been known various water-gel explosives, wherein shirasu hollow microspheres, which are obtained by firing volcanic ash and the like, are used as a density adjusting agent (for example, Japanese patent Laid-open application No. 84,395/81). It is known that each particle of shirasu hollow microspheres forms a single independent bubble or forms a relatively small number of bubble assemblies, each consisting of a secondary particle containing several number of bubbles fused to each other. However, shirasu hollow microspheres are very brittle and are easily broken by a shock and pressure from the exterior, and hence the use of shirasu hollow microspheres is apt to cause the dead pressing phenomenon in the resulting water-gel explosive.
Further, there has been disclosed a method for adjusting the density of W/O explosive without using these hollow microshpheres, wherein a foaming agent or a gas-generating agent is added to the raw material mixture of the explosive or bubbles are blown into the raw material mixture by means of a mechanical stirring at the production of the explosive to contain simple bubbles in the resulting W/O explosive (for example, U.S. Pat. No. 4,008,108). However, these simple bubbles can not be contained in an amount more than a certain amount, are difficult to be contained in a W/O explosive for a long period of time, and leak from the explosive with the lapse of time, and hence the W/O explosive loses its cap-sensitivity and deteriorates in a short period of time, and is difficult to be practically used.
Japanese patent Laid-open applicaiton No. 51,685/85 and No. 90,887/85 disclose the use of a gas-retaining agent having a large particle size or a gas-retaining agent consisting of a multi-bubble body as a density adjusting agent. The use of these gas-retaining agents is a very effective means for obtaining a W/O explosive having a low detonation velocity. The inventors have found out, during the course of the investigation of these gas-retaining agents, that the use of a cushioning medium made of a specifically limited material is very effective for improving the resistance of W/O explosive against dead pressing.
It is a surprising fact that, among the recent investigations for improving the resistance of W/O explosive against dead pressing by increasing the strength of a density adjusting agent, the use of a cushioning medium made of a specifically limited material, that is, the use of a cushioning medium consisting of a soft material, is very effective for improving the resistance of W/O explosive against dead pressing.
The inventors have made various investigations with respect to this phenomenon, and arrived at the present invention.